1. Field of the Invention
This invention relates to an optical fiber cable and more particularly to an optical fiber cable of multi-core single-mode type formed by fusion splicing of the corresponding ends of optical fibers in batch and a method of producing the same.
2. Description of the Related Art
As the demand for optical fiber cables in communication network systems has been increased, there have been developed optical fiber cables each of which comprises serially arranged optical fiber ribbons each having a plurality of optical fibers of multi-core single-mode type arranged in parallel to one after another. For efficient manufacturing purpose the abutting ends of the optical fibers in adjacent optical fiber ribbons are fusion spliced.
Currently, widely known optical fiber cables generally are of either a multi-mode type having a relatively large diameter such as 50 .mu.m or more, or a single-mode type having a relatively small core diameter such as 8 to 10 .mu.m.
End portions of optical fibers of the single-core single-mode type optical fibers are easily fusion spliced by aligning under observation opposed pair of end portions of fibers with each other during fusion. It has been found that single-core single-mode optical fibers can be found to achieve a low optical connection loss across the splice, more specifically an optical connection loss of 0.05 dB or less.
On the other hand, optical fibers of multi-core type are fusion spliced on a fiber splicer in batch by causing the ends of the optical fibers of each optical fiber ribbon to abut against the corresponding ends of the optical fibers in the optical fiber ribbon adjacent to said each optical fiber ribbon without checking whether or not each pair of the end portions of the optical fibers are aligned with each other because of the relatively large number of pairs of optical fibers to be fusion spliced.
As shown in FIG. 1, the end portions of optical fibers 101 exposed from a pair of ribbons are laid in V-grooves 100a formed in a pair of blocks 100 so as to be disposed opposed to each other. Thereafter, the optical fibers 101 on the two blocks 101 are moved toward each other to cause the ends of the optical fibers 101 on one block 100 to abut against the corresponding ends of the optical fibers 101 on the other block 100, and the end portions of the optical fibers 101 are fusion spliced in batch in a short time.
In this fusion splicing of the optical fibers 101 of multi-core type, the opposed ends of the optical fibers in both ribbons are not always accurately aligned because of bending of the end portion of the optical fibers. During the fusion splicing, the opposed end portions of optical fibers 101 on the blocks 100 are attracted toward each other by surface tension exerted on the molten parts of the end portions of fibers such that they, usually at least to some degree, are aligned with each other.
More specifically, in ideal cases, the opposed ends of the corresponding two optical fibers 101 on both blocks 100 to be fusion spliced are normally displaced transversely from each other before fusion splicing, as shown in FIG. 2. As the end portions are molten by the fusion heat produced by arc discharge between electrodes, both the optical fibers 101 are attracted toward each other by surface tension affecting the end portion in an abutted state, as shown in FIG. 3. Finally, the ends of both optical fibers are self-aligned with each other and are connected together, as shown in FIG. 4.
In the optical fibers of multi-mode type having a diameter of 50 .mu.m or more, fusion splicing of the ends of the two optical fibers are approximately ideally performed because of their larger diameter, and in consequence the corresponding ends of the optical fibers in the adjacent optical fiber ribbons can be easily fusion spliced in batch with a low optical connection loss.
With optical fibers of multi-core single-mode type having a diameter of 8 to 10 .mu.m, however, fusion splicing cannot always be performed in an ideal way, although the ends of both optical fibers themselves can be aligned with each other due to surface tension. More specifically, when the ends of the two optical fibers 101 are transversely displaced much before splicing, as shown in FIG. 5, the cores 103 in the respective fibers 101 are deformed and are not aligned with each other after fusion splicing, as shown in FIG. 6, resulting in poor quality of optical fiber cables in respect of large optical connection losses. When, for example, the displacement between the free ends of the cores of corresponding two optical fibers is 12 .mu.m, multi-core single-mode optical fibers used at a wavelength of 1.55 .mu.m exhibit such a large optical connection losses as large as 1 dB.
The conventional method and the difficulty in splicing fine optical fibers are described in "Development of Arc Fusion Splicer for Ribbon Fiber MF-3S by Tsutomu Onodera et al., Fujikura Technical Review 1990, pages 37 to 42. U.S. Pat. No. 4,978,201, to Yamada and Taya, incorporated herein by reference, discloses a method for measuring splice loss of an optical fiber.
It has been proposed that such a large optical connection loss takes place, on one hand, due to the difference of the outer diameters between two optical fibers to be fusion spliced, including the difference occurring from dust attached to the peripheral surface of the optical fibers and, on the other hand, due to the misalignment of the V-grooves formed in both blocks. However, currently manufactured optical fibers have accurate outer diameters and dust on their peripheral surfaces is carefully removed when they are fusion spliced. Further, the V-grooves ln both blocks can be accurately formed so as to be accurately aligned with each other. Accordingly, it has been found that large optical connection loss does not occur from either such previously proposed causes. It is only in conjunction with the present invention that the cause of--and a reliable method of avoiding--such a large optical connection loss has now been discovered.